Due to the ever advancing technical development of modern cranes, and in particular due to developments in the mobile crane sector, which in the context of the present invention includes in particular mobile telescopic cranes and mobile crawler cranes with braced mast booms, so-called lattice boom cranes, both the load capacities and the dimensions of the cranes, in particular the boom lengths of mobile cranes, are increasing significantly. However, this primarily positive development in crane design results in the cable drives that are necessary for hoisting operations of the cranes meeting their technical limits with increased frequency.
Due to the ever increasing carrying capacity of cranes, the tensile forces which a hoisting cable has to withstand are clearly increasing. In the case of unchanged materials characteristics this often leads to an increase in the cable diameter. As a rule this is also associated with a further increase in the diameter of the cable drums or the hoisting winches. Furthermore, due to the ever increasing dimensions of the cranes, in particular of the boom lengths, the cable lengths that have to be stored on the cable drum also increase, which also results in increasing winch diameters or cable drum diameters. These ever increasing winch diameters or cable drum diameters in turn require ever increasing driving torques of the winches; driving torques which at times can now hardly be generated with the usual drive units.
In order to satisfy the need for ever increasing sizes of winch drums or cable drums and the associated increase in driving torques, the use of a so-called double bottom-hook block as shown in FIG. 1 is known. In contrast to a single bottom-hook block, such a double bottom-hook block comprises two separate sheave sets, each of which is a component of two separate cable drives, wherein each cable drive is operated by a separate winch. In order to prevent the double bottom-hook block from assuming an inclined position, for example as a result of unequal cable elongation in the individual cable drives or as a result of not completely accurate synchronous operation of the winches, in other words to prevent the individual cable lines of a cable drive with a double bottom-hook block from being subjected to different loads, such a double bottom-hook block comprises a mechanical load equalisation which couples the two separate sheave sets such that different elongation in the cables or not completely accurate synchronous operation of the cable winches can be compensated for.
However, such double bottom-hook blocks with mechanical load equalisation often prove disadvantageous because, as a result of the increased design height due to the mechanical load equalisation, they directly result in a loss of hoisting height. Furthermore, double bottom-hook blocks always comprise multiple joints, which in particular during slinging and putting down cause problems so that such double bottom-hook blocks overall are relatively unwieldy. Because double bottom-hook blocks have to provide load equalisation for very considerable forces, namely for the sum of the forces from several hoisting-cable lines, these double bottom-hook blocks are often very solid and extremely difficult to handle.
Another known approach to the problem of synchronising two separate winches for the hoisting-cable drive of a crane provides for the two winches to be synchronised directly mechanically, for example by means of a toothed wheel arrangement. It is also known to continuously monitor the angular velocity of two separate hoisting winches and to equalise any difference in speed by changing the angular velocity of at least one of the two winches. Since it is not possible—either by means of mechanical synchronisation or by means of monitoring the angular velocity of the hoisting winches—for example to take into account different cable elongation in the two hoisting-cable lines, this approach to synchronising two hoisting winches is also unsatisfactory. Furthermore, in these synchronisation methods it is not possible to take into account the influence that the cable layer at the time has on the respective cable drum, or to take into account other geometric influences such as for example small differences in the diameters of the cables. Accordingly, cable drives that are operated by means of such known synchronisation methods often have a tendency to stress the individual cable lines unevenly, which at times in extreme cases can lead to overloading.
In U.S. Pat. No. 5,579,931 A an improved method and system for a liftcrane in which a load is lifted through the combined action of first and second hoist drums are disclosed. The known method and system use a first rope wound on one hoist drum and a second rope wound on the second hoist drum. The ends of the ropes opposite the hoist drums are linked together to transmit tension between them. The load is coupled to the ropes. If the take up speed of one of the ropes exceeds the take up speed of the other, the linked ends of the ropes will shift. This condition is detected and the operation of at least one of the first and second hoist drums is modified to bring the take up rates into balance. This system is advantageously used with a hoist block sheave arrangement. This system can also be used with a single rope in which each of the ends of the single rope are wound on a separate one of the hoist drums and the load is coupled to the middle of the rope.
U.S. Pat. No. 6,651,961 B1 discloses a multi-block rigging system for a heavy crane, pulling or lifting device. The system uses sheave blocks in series orientation to enable the use of standard, economical or preferred, size winch drums and standard, economical or preferred, diameter and length wire rope, each forming a separate set of reeving lines. Each set of reeving lines moves its corresponding load block a proportional distance of the total travel length for the load hook. Alternatively, different line parts of line for each reeved set enables different travel speeds of the load block for different capacity requirements.
In DE 34 04 505 A1 a length compensation between two ropes in a rope drive, in particular for cranes, is disclosed. Here, the free ends of two ropes are in each case attached to two jokes, from which at least two rope sections are passed over compensating pulleys, the two rope sections consisting of a right-hand and a left-hand wire rope.
DE 41 30 970 A1 corresponding to U.S. Pat. No. 5,377,296 A discloses a control system for an electric motor arranged to drive a rope drum of a mine winder or a hoist system which includes a conveyance supported by a rope and which forms an oscillating system. The known control system includes a load sensor which monitors the load in the rope and a rope length sensor which monitors the length of rope paid out from the rope drum. A motor control unit is responsive to signals from the sensors and calculates set points for speed, acceleration and jerk of the oscillating system. The control unit generates a control signal which is related to a natural oscillation mode of the oscillating system so as to prevent the excitation of oscillations in the system, and controls a motor drive in accordance with the control signal.
A further control and hydraulic system for liftcrane is known from EP 0 422 821 B1 corresponding to U.S. Pat. Nos. 5,189,605 A, 5,297,019 A, and 5,579,931 A. This known lift crane includes controls by which an operator can run the lift crane and mechanical subsystems each powered by a closed loop hydraulic system having a pump and an actuator. According to this known system a controller responsive to the controls and connected to the mechanical subsystems is provided, and further the controller is capable of running a routine for controlling said mechanical subsystems to define operation of the lift crane.